Programmed cell death, or apoptosis, is employed by multicellular organisms to eradicate physiologically-divergent cells that threaten development, homeostasis and overall survival. Disruption of the apoptotic cycle can lead to a number of life-threatening human disorders including cancer, immunodeficiency, autoimmune and neurodegenerative diseases. Historically, one focus of cancer research efforts has been centered on elucidation of regulatory mechanisms and identification of proteins responsible for programmed cell death in normal and transformed cells. Comparative analyses have illustrated the effects of various diseases and cancerous tumors on apoptotic dysfunction and, hence, have uncovered target proteins for therapeutic intervention.
A variety of pathways are available to initiate programmed cell death. The two main pathways are i) the extrinsic pathway, which relies on activation of cell surface death receptors by extracellular signals, and ii) the intrinsic pathway, which is dependent on release of cytochrome c from mitochondria as a result of cellular DNA damage or loss of survival signals. Both pathways activate a family of cysteine proteases, known as caspases, that specifically target and degrade vital cellular proteins for nuclear membrane and DNA fragmentation, chromatin condensation and eventual cell death. Caspases are heavily regulated proteins due to the extreme significance of their activity, as inappropriate activation can have devastating effects for the organism. Therefore, all caspases are synthesized as inactive procaspases for which activity is induced upon proteolysis of their maturation cleavage site and further regulated by specific intracellular protein inhibitors. Many cancers have been linked to deficiencies in caspase function and, as a result, represent an important class of drug targets for anti-neoplastic design. In addition, other disease states can be targeted by this class of drug.
Apoptotic caspases are divided into “initiators” and “executioners”. The extrinsic and intrinsic apoptotic pathways utilize independent initiator caspases 8 and 9, respectively. Once activated, the initiator caspases 8 and 9 converge to activate executioner caspases 3, 6 and 7. The executioner procaspases 3, 6 and 7 represent a class of proteases believed solely responsible for the last step of the cellular apoptosis cascade by cleaving many proteins including actin, nuclear lamin and various regulatory proteins. Cancerous tissues have been shown to express elevated levels of the executioner procaspases and thus, represent an important target for anti-neoplastic intervention. Specifically, activation of executioner procaspases by small molecule agonists would promote cell death in lieu of upstream signaling cascades and cellular apoptosis inhibitors.
However, targeting members of the caspase family for therapeutic design has been a difficult endeavor as evidenced by the complete lack of caspase-directed therapies. Drug discovery efforts have been hampered by the stringent preference of all caspase active sites for a substrate electrophilic carbonyl and aspartyl functionality, thereby preventing diffusion of small molecule inhibitors across the cellular membrane during drug administration.
Therefore, there is a need to develop small molecule compounds and methods that are capable of specifically activating executioner procaspase 3, 6 and 7 both in vitro at physiological concentrations and promoting cellular apoptosis in vivo as well as exhibiting activity for treating various diseases including cancers and neoplastic diseases. The present invention meets this and other needs.